Beech V35A N5470U accident near Attalla, Alabama

Probable cause

A loss of control inflight and the failure of a component of the up elevator flight control system due to fatigue, which rendered recovery from a right descending turn improbable.

NTSB analysis

The pilot was flying at 9,000 ft mean sea level (msl) on an instrument flight rules flight plan, and likely in turbulent instrument meteorological conditions, when the airplane entered a right descending turn. As the airplane descended through 7,000 ft msl, the controller broadcast the airplane’s call sign and the pilot replied, “yeah im with you im” but the rest of the comment was unintelligible. The airplane continued the right descending turn with the radius of turn becoming smaller until the airplane impacted an open field at a high rate of descent.

Postaccident examination determined that the “up” elevator control cable assembly failed after a terminal end in the turnbuckle at the tail of the airplane fractured from fatigue. The presence of fatigue cracking at this location would be consistent with failure of the elevator control cable and subsequent loss of control of the aircraft. The initiation of multiple fatigue cracks was consistent with the roughened surface and pitting due to widespread corrosion. The remnants of cadmium (Cd) found in the iron oxides embedded in the cracks was indicative of a previous Cd coating on the steel surface of the turnbuckle. The Cd was meant to protect the underlying steel from corrosive attack; however, the Cd surface coating had completely deteriorated, leaving the underlying substrates vulnerable.

Although an inspection of the airplane, including the flight control cables, was performed as part of an annual inspection about 7 months before the accident, the location of the turnbuckle in the airframe likely would have made it difficult to thoroughly inspect the component while still installed in order to identify wear of the cadmium layer or surface cracks in the part.

Given the pilot’s flight and medical history, it is unlikely that his previous amputation or medications contributed to the accident. It is likely that the flight encountered turbulence during the flight. Given the evidence available, it could not be determined if the turbulence or the failure of the turnbuckle was the initiating event of the right descending turn or if the fatigue-weakened turnbuckle failed as the pilot was attempting to recover from the descending turn. After the turnbuckle failed, the pilot’s ability to control the airplane’s pitch would have been significantly diminished and would have made recovery to a nominal flight attitude improbable.

Text from the NTSB. Open the NTSB docket for ERA21LA068 for the full record.